1. Field of the Invention
The present invention relates to methods of manufacturing alkaline storage batteries, and more particularly to methods of manufacturing superior porous metal plates for positive electrodes.
2. Background Art
With the increasing trend for equipment to be portable and cordless, the need for small and light secondary batteries with high energy density for use as power sources for this type of equipment has grown. There is thus a strong and growing demand for inexpensive secondary batteries with high capacity. Accordingly, there is downward pressure on prices of alkaline storage batteries, typically nickel-hydrogen storage batteries and nickel-cadmium storage batteries, combined with calls for improved reliability.
The steps of manufacturing conventional alkaline storage batteries are described next. First, an electrically insulated separator is interposed between the positive and negative plates, mainly using nickel hydroxide as an active material, to create a laminated member. This laminated member is wound spirally to create an electrode plate group. Then, the electrode plate group is housed in a metal battery case, and alkaline electrolyte solution is filled to a predetermined level. Lastly, the top of the battery case is sealed by a top cap which also acts as a positive or negative terminal.
The steps of manufacturing a conventional positive plate are described next. Active paste is made by mixing the active material mainly made of nickel hydroxide, water, and water-soluble binder. This active paste is impregnated in a porous nickel plate, and then dried. After drying, the porous nickel plate is pressed by passing it between small rollers to make the plate thickness uniform and enhance the density of the impregnated active material. Cracking inevitably occurs during winding in some of conventionally manufactured positive plates, with the risk of cracking rising as the battery capacity becomes greater.
To enable the battery to discharge a high current, the area of facing positive and negative plates need to be made as broad as possible during winding. But the broadening of the area causes increase of the volume of the core member. The Japanese Laid-open Patent No. H3-226969 shows how to suppress cracking which may occur while winding positive and negative plates, where a method of peeling is adopted in which the twisting direction of electrode plates becomes perpendicular to the deflating direction of pores during foaming.
The positive plate using a conventional material is compressed on an inner periphery, that is, to the winding core, but expanded on the outer periphery. This is the main cause of cracking on the outer periphery. In particular, batteries with high electrical capacity have insufficient flexibility, increasing the risk of causing cracking. A cracked portion as occurred above penetrates through the separator, contacts the negative plate, and causes internal short-circuiting.
Moreover, foamed urethane, which is the base material for the porous positive plate, cannot be made thinner than 1 mm due to the performance limitations of current foam cutting machines and peeling machines. Accordingly, a core member having a two-dimensional structure such as a punch metal is required to be used in order to achieve a core thickness below 1 mm.
If the active material is applied to this two-dimensional core member, a conductive net with the active material is not sufficiently established, causing difficulty in drawing out the required battery characteristics.
Furthermore, if a porous metal plate of 1 mm or thicker is rolled for thinning, the skeleton is broken, pores on the surface are blocked, or impregnation of mixed paste of active material is deteriorated in some cases.